The present invention relates generally to a switching element for a memory device and a method of manufacturing the same, and more particularly, to a square pillar-shaped switching element for a memory device and a method of manufacturing the same.
Recently, research has been conducted with the hope of developing a novel memory devices which has the characteristics of a nonvolatile memory device, can be highly integrated, and has a simple structure. One group of novel memory devices studied is the phase-change memory device.
Generally, in a phase-change memory device a phase-change film is interposed between upper and lower electrodes. The phase change film is reversible changed from an ordered crystalline solid state to a disordered amorphous solid state by the flowing of current between the upper and lower electrodes. The phase-change memory device can thus serve as a memory device by exploiting the variable physical properties, e.g., a variable resistance, of the phase-change film to distinguish the information stored in cells. For example, the difference in resistance between the crystalline phase-change film and the amorphous phase-change film can be used to determine the logic level of information stored in cells.
One of the most important design factors that must be given consideration when developing a phase-change memory device is the minimization of a programming current. One method of reducing programming current is to substitute transistors with vertical type PN diodes for use as a cell switching element. Vertical type PN diodes are considered advantageous for their use as a cell switching element since the vertical type PN diodes enable current to easily flow when compared to transistors, thereby resulting in a reduction of programming current. Further, vertical type PN diodes enable a reduced cell size, and thus the application of the vertical type PN diodes facilitates high integration of the phase-change memory device.
However, a conventional phase-change memory device employing PN diodes as a switching element is problematic in that parasitic bipolar junction transistors are formed between the PN diodes and a P-type substrate, thus incurring the loss of driving current.
Further, the conventional phase-change memory device comprising the PN diodes has a structure in which PN diodes are electrically interconnected through an N+ region formed on a surface of an active region. As a consequence of this structure, the resistance of the N+ region becomes high, causing variations in the driving currents between cells, and thus it is required to improve the design and process thereof.
Moreover, the conventional phase-change memory device comprising the PN diodes is problematic in that its manufacturing process is complex, because complicated unit processes, such as an epitaxial process and the like, must be performed in order to form the PN diodes.
Furthermore, in the conventional phase-change memory device comprising the PN diodes, word line contacts for connecting word lines to a silicon substrate are formed in order to solve the problems attributable to the high resistance of the silicon substrate. However, this causes an increase in the number of processes required to manufacture the phase-change memory device and an increase in the area consumed by the phase-change memory device, thereby decreasing economical efficiency.
Meanwhile, in order to solve the problems attributable to high resistance of the silicon substrate, Schottky diodes are used as a switching element of a phase-change memory device. The phase-change memory device using the Schottky diodes as the switching element can decrease the resistance of the silicon substrate by three fold.
These Schottky diodes are formed in holes formed using a hole-type mask in the form of a round column. However, in a memory device to which 4F2 technology is applied, it is difficult to form small-sized Schottky diodes.
In other words, with the increase of high-integration of a memory device, it is very difficult to realize small-sized holes, which are Schottky diode formation regions, through an exposure process. Further, the distance between the holes is decreased, whereby the margin of a photosensitive film is also decreased, causing a hole patterning process to gradually become more difficult.
In addition, since the holes, which are Schottky diode formation regions, are formed through an insulating film patterning process, there are problems in that the holes cannot be stably formed in desired regions and can instead be formed in a misaligned state, and in that the characteristics of the Schottky diode can be deteriorated.